The present invention relates generally to a system, apparatus and method for filtering acoustic noise within a fluid flow system. More particularly, the invention relates to the mitigation or attenuation of ultrasonic noise, and the incorporation of such an apparatus within a fluid flow system that includes a noise source and an ultrasonic device. Further, the invention relates to the elements or components of such an apparatus, particularly directed to attenuating direct and/or indirect noise within a fluid flow environment.
Sound or noise is a longitudinal mechanical wave motion in an elastic medium and is classified according to its frequency—infrasonic, audible, and ultrasonic. The infrasonic classification refers to frequencies below the detection level of the human ear (less than 20 Hz). The audible classification refers to frequency that can be detected by the human ear (from 20–20,000 Hz). The ultrasonic classification refers to frequencies above the detection level of the human ear (above 20,000 Hz). Sensory effects of sounds denoted by a physiologist as loudness, pitch, and quality are correlated with the measurable parameters of sound denoted by physicists as intensity, frequency and wave shape.
The intensity of a sound wave is the amount of wave energy transmitted per unit time per unit area normal to the direction of sound propagation. That is, the intensity of sound is the power transmitted per unit area. In the audible classification of noise, the significant intensities for a human species are:
NoiseIntensity (W/M2)Level (dB)Hearing Threshold1 E-120Whisper1 E-1020Conversation1 E-0665Street TrafficI E-0575Train in a Tunnel1 E-02100Pain Threshold1 E-00120
Acoustics is a systematic investigation of the nature, origin, and propagation of sound. Acoustic noise generation in a closed conduit can occur from many sources, including protruding gaskets, misaligned pipe flanges, headers, line size changes, valves, etc. The flowing velocity in the pipe is a major factor in the character of the acoustic noise. When pipe velocity is below 50 fps, one would expect noise from only a control valve, which is designed to manipulate the flow. When the pipe velocity is greater than 50 fps, noise generation can be initiated by a multitude of the aforementioned sources.
Sound waves are pressure pulses propagate in accordance with acoustic plane wave theory. Sound propagates as a pressure wave in gas flow system, i.e., pipe system or other fluid conduit, at the velocity of sound of the fluid. Such a propagating pressure wave will be reflected and absorbed at impedance discontinuities forming standing wave patterns or acoustic resonances. These resonances typically amplify pulsation by a factor of 10 to 100.
Noise travelling along the longitudinal direction, i.e., in parallel with the longitudinal centerline of the conduit, may be referred to as direct noise. Noise travelling in a direction that is not parallel to, or not oblique with respect to, the longitudinal centerline may be referred to as indirect noise. Direct noise may become indirect noise upon encountering a bend, obstruction, or certain discontinuities in the flow medium, that forces the pressure wave to reflect, refract or otherwise deviate from the direct or longitudinal direction. Indirect noise may, therefore, propagate through a extensive run of conduit by bouncing or reflecting off the walls of the conduit.